1. Field of the Invention
The present invention relates to zoom lenses, and more particularly, to a lens-driving device used for driving an optical zoom lens called an inner-focus zoom lens or a rear-focus zoom lens.
2. Description of the Related Art
Inner focus (or rear focus) zoom lenses in which a lens unit other than a front lens unit is moved for focusing are known.
In a typical inner focus zoom lens, the size of the overall lens apparatus can be reduced by reducing the diameter of the front lens unit at the object side. In addition, the speed of autofocus (hereafter abbreviated as AF) can be increased by reducing the weight of a focusing lens. In addition, the hood effect and the operability of a filter can be improved by using a non-rotating front lens unit.
In general, in an inner focus zoom lens, the amount of movement of the focusing lens must be varied in accordance with the change in the focal length occurring in a magnification-changing operation. Accordingly, as described in, for example, Japanese Patent No. 2561350, the amount of movement of the focusing lens is automatically corrected in accordance with the change in the focal length.
However, corresponding to an increase in the zoom magnification, new problems have arisen. In a typical inner focus zoom lens, when the object distance is constant, the amount of movement of the focusing lens gradually increases from the wide-angle end to the telephoto end. In particular, in high-magnification zoom lenses (zoom lenses with a magnification of 5× or more), the amount of movement of the focusing lens differs greatly between the wide-angle end and the telephoto end, and the amount of movement rapidly increases as the zoom position approaches the telephoto end.
When a method disclosed in the above-mentioned Japanese Patent No. 2561350 is applied to a high-magnification zoom lens having such characteristics, the amount of zoom parameter shift caused by the optimization of a focus cam (to achieve a single, smooth cam by combining the movement trajectories of the focusing lens at different focal lengths) is increased. Therefore, the inclination of the movement trajectory of the zoom lens differs greatly between the wide-angle end and the telephoto end. This causes an operability problem that the operating torque increases or rapidly changes in the magnification-changing operation or a problem that a change in focus that is out of the allowable range must be tolerated in the magnification-changing operation.
Accordingly, there are demands for lens-driving methods for sufficiently reducing the change in focus in the magnification-changing operation without degrading the operability in the magnification-changing operation. For example, according to Japanese Patent Laid-Open No. 2001-188161, the engagement position and the engagement range of a focus driving cam and a follower provided on a focusing lens at each focal length are corrected by changing the moving speed of the engagement position of the focus driving cam and the follower in accordance with the focal length.
According to the technique disclosed in the above-mentioned Japanese Patent Laid-Open No. 2001-188161, a speed-changing mechanism is used for changing the moving speed of the engagement position of the focus driving cam and the follower provided on the focusing lens in accordance with the focal length. The speed-changing mechanism includes a focus driving member having a key groove that engages with a key follower provided on the focusing lens and that is formed in a non-straight cam shape such that at least a portion of the key groove is not parallel to the optical axis. The focus driving member rotates the focusing lens in the focusing operation due to the engagement between the key groove and the key follower.
The focus driving member included in the speed-changing mechanism does not move along the optical axis. Therefore, in the case in which the movement trajectory of the focusing lens in the magnification-changing operation passes through the same position along the optical axis a plurality of times, the moving speed of the engagement position of the focus driving cam and the follower provided on the focusing lens must be the same at all such positions. Therefore, it is difficult to smoothly correct the engagement position and the engagement range of the focus driving cam and the follower provided on the focusing lens at each focal length.